TreeDataGrid: Support frozen columns on the right side

[ITDancer13]

TreeDataGrid currently supports frozen columns on the left side of the viewport, but not on the right.

It would be useful to support freezing columns on the right edge of the viewport as well, so they remain visible while the rest of the grid scrolls horizontally.

One example would be an actions column (e.g. a ... menu), but the feature would be generally useful regardless of the column content.

As discussed on Telegram: Since RTL layouts can mirror left/right semantics, something direction-aware like: FrozenColumnCountStart and FrozenColumnCountEnd.